ST90158 - DEVICE ARCHITECTURE
SYSTEM REGISTERS (Cont’d)
If the bit is set, data is accessed using the Data
Pointers (DPRs registers), otherwise it is pointed
to by the Code Pointer (CSR register); therefore,
the user initialization routine must include a Sdm
instruction. Note that code is always pointed to by
the Code Pointer (CSR).
specifies the location of the lower 8-register block,
while the srp0and srp1instructions automatical-
ly select the twin 8-register group mode and spec-
ify the locations of each 8-register block.
There is no limitation on the order or position of
these register groups, other than that they must
start on an 8-register boundary in twin 8-register
mode, or on a 16-register boundary in single 16-
register mode.
Note: In the current ST9 devices, the DP flag is
only for compatibility with software developed for
the first generation of ST9 devices. With the single
memory addressing space, its use is now redun-
dant. It must be kept to 1 with a Sdminstruction at
the beginning of the program to ensure a normal
use of the different memory pointers.
The block number should always be an even
number in single 16-register mode. The 16-regis-
ter group will always start at the block whose
number is the nearest even number equal to or
lower than the block number specified in the srp
instruction. Avoid using odd block numbers, since
this can be confusing if twin mode is subsequently
selected.
2.3.3 Register Pointing Techniques
Two registers within the System register group,
are used as pointers to the working registers. Reg-
ister Pointer 0 (R232) may be used on its own as a
single pointer to a 16-register working space, or in
conjunction with Register Pointer 1 (R233), to
point to two separate 8-register spaces.
Thus:
srp #3will be interpreted as srp #2 and will al-
low using R16 ..R31 as r0 .. r15.
For the purpose of register pointing, the 16 register
groups of the register file are subdivided into 32 8-
register blocks. The values specified with the Set
Register Pointer instructions refer to the blocks to
be pointed to in twin 8-register mode, or to the low-
er 8-register block location in single 16-register
mode.
In single 16-register mode, the working registers
are referred to as r0 to r15. In twin 8-register
mode, registers r0to r7are in the block pointed
to by RP0 (by means of the srp0 instruction),
while registers r8to r15are in the block pointed
to by RP1 (by means of the srp1instruction).
The Set Register Pointer instructions srp, srp0
and srp1 automatically inform the CPU whether
the Register File is to operate in single 16-register
mode or in twin 8-register mode. The srpinstruc-
tion selects the single 16-register group mode and
Caution: Group D registers can only be accessed
as working registers using the Register Pointers,
or by means of the Stack Pointers. They cannot be
addressed explicitly in the form “Rxxx”.
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